This invention relates to an improved intraocular surgical instrument which can be used with microsurgical scissors, forceps, knives and the like.
The invention will be described in connection with its preferred usage and that is a microsurgical scissor. Microsurgical scissors are in widespread use during intraocular operations by surgeons worldwide. Three types of driver (actuation) systems for these scissors are in current use: manually operated handles with squeeze-type or lever depression actuation, pneumatic piston linear drivers, and electrical motors of direct current or solenoid drive.
Grieshaber and Co., A.G. of Switzerland produces a wide variety of microsurgical scissors and driver systems including the Proportional Control System (PCS), a Membrane Peeler Cutter (MPC) and manual drive handles (Southerland Style). Pneumatic drivers are produced by Storz Instrument Co. of St. Louis, Mo.; Alcon Surgical, Inc. of Ft. Worth, Tex.; The Dutch Ophthalmic Research Corporation (DORC) of Holland; and TREK Medical of Muckwamago, Wis. Each company provides various scissortips attachable to these drivers, or allow them to cross adapt to other manufacturer's designs. Manual handles are provided by Grieshaber, Alcon, Storz, DORC, TREK and others.
Most intraocular scissors have design similarities in which a pair of cutting blades extend from the end of a tubular needle with one blade being fixed and the opposite opposing blade end being reciprocated between an open and closed position with respect to the fixed blade. This reciprocating movement is accomplished through the action of one of the three above-listed driving systems, i.e. manual, pneumatic or motor.
In the manual driver, actuation of one blade end against the other is through the transfer of movement to the movable blade by depression of a single lever extending from the handle (Southerland-Grieshaber) or by squeezing two opposing platforms on opposing sides of the handle. The movable blade moves through an excursion of 60 to 70 mils (0.060 to 0.070 inch) from the fully open to the fully closed portion during actuation.
In the pneumatic driver, actuation is achieved by pressurizing a piston with a compressed gas source into a chamber within the handle, which causes the piston to move forward against a spring, moving the one blade against the other, closing the blades. Opening the blades is accomplished by movement in the opposite direction through energy stored in the spring, as the gas within the piston chamber is released. Control of the gas pressure release to the piston is accomplished by depression of a foot pedal by the surgeon. Scissor actuation is thus accomplished via footpedal control rather than via finger control, allowing the surgeon to hold the instrument steady without inducing any unnecessary tremor or motion to the blades due to finger movement. The footswitch is a linear depression switch which also allows selection between a "proportional cut" mode versus a "multicut" mode by the manufacturer. In the "proportional cut" mode, the scissors blades close at a rate and position directly related to the rate and position of depression of the pedal, e.g. one-half depression of the pedal will close the scissors half way, full depression will close the scissors fully, etc. The scissors will move open and closed inducing a cutting force only as the footpedal is depressed and released, with a more rapid depression resulting in a more rapid closure, etc. A full depression of the footpedal by the surgeon closes the blades to reduce the width of the scissor blade profile so that it can be inserted through a small slit, e.g. 1.0 mm slit, in the eyeball. Once within the eyeball, the scissor blade can be opened and closed by the footpedal action described above. To remove the scissors from the eyeball, the blades are fully closed by complete depression of the footpedal to again reduce the scissor width. In the event of failure of the piston driver while it is in the eyeball, the surgeon can manually close the scissors in this emergency situation by screwing a thumbscrew down on the driver to allow its safe removal from the eye. While the scissors are in the eye, the surgeon can select the "multi-cut" mode by the temporary lateral motion of the footpedal. Depression of the pedal in this mode causes movement of the blade from its open to its closed position and back to its open position repetitively at a rate of approximately one stroke per second. A slight depression of the footpedal activates this multiple repetition mode, which continues at the same rate regardless of the amount of depression to the pedal. The surgeon returns to the "proportional cut" mode and fully depresses the pedal to fully close the scissor blades and holds them closed to remove them from the eye.
Motor drivers of either rotary or linear solenoid style activate scissor closure by controlled transfer of the motor energy to the movable blade. The MPC is an automated solenoid-style microscissors that has a nondetachable pair of cutting blades extending from the end of a tubular needle, with the outer blade end being fixed and the inner blade end being reciprocated between an open and closed position with respect to the fixed blade. A first footswitch is operated by the surgeon to move the movable blade to a closed position reducing its profile allowing its introduction into the eye. Once inside the eye, the first footpedal is released and the movable blade snaps open due to energy stored in a spring within the driver handle. Depression of a second footpedal causes the blades to move from an open position to a closed position against a spring, and then back to an open position. The scissors always default to an open position during activation of the second footpedal. The blade excursion is again 60 to 70 mils (0.060 to 0.070 inch) and travels at a rate of 1000 mm/sec from the open to the closed position. The moving blade cuts in about 5 milliseconds and remains shut for about 15 milliseconds before automatically returning to the open position. The MPC can also be operated in a "single cut" mode versus a "multicut" mode. In a single cut mode, depression of the second footpedal results in one excursion of the movable blade and one resulting cut. Release and redepression of the second footpedal is required to initiate a second excursion and cut. In the "multicut" mode, hereinafter referred to as an oscillation or oscillatory mode, the blade moves through a series of repetitive cuts or oscillations at a rate of one to five strokes per second, with each stroke traveling at 1000 mm/sec. These oscillations continue While the second footswitch is held in the depressed position. This MPC microsurgical scissor is gas sterilizable and is not recommended to be steam autoclaved, except in "emergency situations", as would be desirable for an intraocular scissor.
In the PCS-Grieshaber system, there is a power operation and control of a variety of Southerland intraocular instrument tips including a scissor by energy from a DC motor within the handle. This PCS system includes movement of the cutting blade in either the "single" stroke cutting mode or a continuous oscillatory motion mode. Manual selector switches allow choice between these modes, as well as selection of the rate of scissor closure for either mode, and the rate of oscillations in the oscillatory mode. Additionally, a manual dial switch allows the opening distance between the radius of curvature of the blades to range from 1/3, 2/3 or full.
In the MPC automated microscissors and in some other Southerland and manual scissors, the scissor tip is of the vertical design in which the outer fixed blade has a cutting edge substantially parallel to the cutting edge of an inner movable blade such that the cutting edges cut on a substantially straight line on a guillotine principle. The surfaces are not perfectly parallel, however, and do have some angle between them, creating a cutting point where the blades are in contact. The MPC scissors close so quickly, however, that their cutting point is effectively a straight line rather than a single point as in conventional angled scissors described below.
Shear is a force responsible for division of the tissue held within the scissor blades regardless of their design, and describes a vector perpendicular to the vector of movement of the direction of closure to the blades. The strength of the force vector pushing the blades one against the other is responsible for the creation of the shear force vector.
Guillotine or parallel blade scissors tend to crush the tissue between the blades before the shear begins to divide the tissue. This crush action has an advantage of holding the tissue within the blades and preventing forward thrust of the tissue out from the blades. It has a distinct disadvantage, however, of creating crush artifact in the tissues due to tissue deformation that occurs prior to its shearing, as illustrated in FIGS. 25 and 26 hereinafter. Due to motion of the scissors between cuts and the inability to begin a cut immediately in the exact same position as the ending of the previous cut, the tissue is engaged in a slightly different location, resulting in steps or shoulders between cuts and also in curved or scalloped surfaces on the cut tissue wall, as shown in FIGS. 25 and 26. Parallel or vertical-style scissors manually driven exhibit the same tissue sectioning artifacts as the MPC microsurgical scissor and is shown in FIG. 22.
Angled or horizontal-style scissors can be manually or automatically driven depending on the manufacturer, but the cutting characteristics are similar, and different from vertical scissors. In angled scissors, the blades pivot from a fulcrum point and create a single cutting point where the blades are in contact. When the blades are fully open, this point is closest to the fulcrum and successively moves forward down the scissor blades toward the tip as the scissor closes. The blades also become relatively more parallel as they close and begin to induce some crush action near the tip.
As the forward movement of the cutting point proceeds toward the tips of the blades it induces a forward thrusting motion to the tissue due to the resistance of the tissue being sheared, thus serving to push the tissue ahead of the scissor as it closes. Also, a greater area of tissue is included between the blades during closure, further increasing tissue resistance and thus contributing to increased forward thrust. Forward thrust of tissue during ocular surgery is annoying and clinically undesirable as it contributes to irregular cuts and longer procedures. Needing to "chase" the target tissue puts additional traction on surrounding normal tissue and contributes to tears and accidental cuts within the normal tissue.
With a force applied about the fulcrum of the scissors blades the shear forces are at a maximum when the cutting point is nearest to the or fulcrum point of the blades. The blades will stay together with the greatest force near the pivot and thus shear is maximum here. Further, less force is required to close the blades to create this shear force when the cutting point is near the pivot. As the cut proceeds and the cutting point moves farther from the pivot, mechanical advantage is lost, greater forces are required to close the blades, shear force is lost and the scissor blades may actually be pushed apart at the tips if the tissue resistance force becomes greater than the shear force.
In order to maintain a force pushing the blades together at the cutting points, the blades are positioned against each other by two opposing forces called camber. The radius of curvature of the camber increases along the length of the blade, to create more shear force at the tip to try to overcome some of the loss of shear force due to the loss of mechanical advantage, as shown in FIG. 29.
In cross section, scissor blades are actually asymmetric wedges opposed to each other, and each want to drive into the tissue at an oblique angle called the "preferential wedge path" (FIG. 30). The camber and closing movement of the blades want to drive the blades into the tissue 60.degree. to 90.degree. away from the wedge path (FIG. 31). The resulting actual movement of the blade is more of a twisting motion as shown in FIG. 32. Because the tissue has resistance to shear, it too will be twisted, more or less by the twisting motion of the blades with softer and thicker tissues twisting and deforming more than harder and thinner tissues. The resulting cross sectional cut has an "S" shape rather than being perpendicular to tissue surface, as shown in FIG. 33. Ocular tissues are usually soft enough to result in "S" shape cuts by conventional scissors, which is less desirable due to its irregular surface, as shown in FIGS. 23 and 24.
As more of the blade surface becomes buried within the tissue during the cut, the lateral resistance increases, preventing a side-to-side motion or a "steering" redirection to the scissors. If the scissors are wide open, the lateral resistance is at a minimum and the scisors can be steered to a new location without distortion induced by lateral resistance.
Every microsurgical scissor currently available for ocular surgery has more or less of the following disadvantages dependent on its individual design: (1) forward thrust during closure pushes the target tissue out of the scissor because of forward motion of the cutting point; (2) crush artifact deforms the tissue during shear; (3) irregular "S" shaped cross-sectional cuts occur, the severity of which is determined by individual ocular tissue characteristics; (4) loss of mechanical advantage during closure causes loss of shear, creating tissue incarceration at the tips, and resultant lateral crush artifact; and (5) increased lateral resistance during closure prevents re-direction of the scissor along a curved line without creating crush and distortion artifact into the cut.